Physics- Filament Lamps, Diode + LED's, Thermistors + LDR's Flashcards
What is an Ohmic conductor?
An Ohmic conductor follows Ohm’s law. It is a component that has a constant resistance. This means that the current flowing through it and voltage across it are always proportional so when plotted on an I-V graph, a straight line through the origin is obtained. This applies to positive AND negative currents as there is no difference if the current flows through in the opposite direction.
What is a non- ohmic conductor?
A non - ohmic conductor is basically the opposite of an ohmic one. It’s resistance varies with the applied voltage or current, so it doesn’t follow Ohm’s law.
List two examples of Non- Ohmic conductors:
-Filament lamp- at higher voltages, the filament will get hot.
-Diodes and LED’s - these components only allow current to pass in one direction only.
LED’s work in the same way and also emit light energy.
Describe the graph of a filament lamp (non- ohmic conductor) :
-At first, the lamp obeys Ohm’s law with a straight line through the origin,
-but then the lamp starts to flatten, suggesting an increasing resistance.
-This is because the filament’s temperature increases; there is more friction between the free electrons, therefore it is harder for them to flow.
It doesn’t matter which way the current flows through the lamp, therefore the same trend is shown for negative voltages.
Describe the graph of Diode (non- ohmic conductor) :
-A diode is like a turnstile,
if the current flows through the wrong way, it experiences a lot of resistance
-but when the current flows through the correct way, there is very little resistance.
-This is because diodes are made from a semi- conductor material, which allows current to flow easily once a small initial voltage (about 0.6V) has been exceeded.
List two more non - ohmic components not the lamp and the diode:
1) Light Dependent Resistors (LDR’s) -This component’s resistance is affected by the light intensity of it’s surroundings.
2) Themistors - This component’s resistance is affected by the temperature of it’s surroundings.
Rather than using an ammeter and voltmeter to THEN calculate the resistance what can we use to directly calculate resistance?
What is it’s advantage?
Rather than using an ammeter and voltmeter to THEN calculate the resistance, we can use a digital multimeter to give a direct measurement of resistance. It’s advantageous as since the device also has it’s own power supply, there is no need to use a separate power pack for the circuit- simply connect the component to the meter to read the resistance.
Write the Method for the required practical: Resistance in a Filament lamp
METHOD
Set up the circuit shown in the diagram.
1)Remember to connect the ammeter in series with the lamp, and connect the voltmeter in parallel with the lamp.
2)Before starting the experiment, check that the meters are both connected properly- they should both give positive readings.
3)Set the variable resistor to it’s lowest resistance value, then switch on the power supply; take the readings for the current through the lamp and the potential difference , across the lamp.
4)Adjust the variable resistor slightly, to obtain and record another pair of current and PD values; repeat this procedure until you have eight pairs of values.
RESULTS
-Draw a results table with 2 columns- one showing potential difference (in volts), and the other showing the current (in amperes)
-Plot a graph, showing current (y-axis) against PD (x-axis); both axes must start from zero.
-Connect the points using a smooth curve of best fit, which must start from the origin.
Write the Method for the required practical: Resistance in a Diode
METHOD
Set up the circuit shown in the diagram.
1) Initially, ensure that the positive terminal on the diode is connected to the positive terminal on the power supply.
The additional (fixed) resistor is necessary to ensure that there is not too much current flowing through the circuit.
2) Remember to connect the ammeter in series with the diode, and connect the voltmeter in parallel with the diode
3) Before starting the experiment, check that the meters are both connected properly- they should give either both positive or both negative readings.
4) Set the variable resistor to it’s lowest resistance value, then switch on the power supply; take the readings for the current through the diode/ LED and the potential difference across it.
5) Adjust the variable resistor slightly, to obtain and record another pair of current and PD values; repeat this procedure until you have eight values.
6) Disconnect the power supply, and swap over the two leads (this will make the current flow around the circuit in the opposite direction through the diode/ LED). Repeat the above experiment.
RESULTS
-Draw a results table with 2 columns - one showing PD (in volts) and the other showing the current (in amperes), showing all positive and negative values.
-Plot a graph, showing current (y-axis) against PD (x-axis), taking care to ensure that both positive and negative values are plotted correctly. Connect the points using a smooth curve of best fit, which must pass through the origin.
What may LDR’s and Thermistors be used for?
LDRs are used in circuits with light sensors.
e.g. a security light or garden light will switch on when the resistance of its LDR is high enough, and switch off when the resistance is low enough
Thermistors are used in circuits with temperature sensors.
e.g. a central heating system or water heater will switch on when the thermistor’s resistance is high enough, and switch off when the resistance is low enough.
Write the METHOD for the required practical: Resistance In a Light Dependent Resistor
In this experiment, a digital multimeter will be used to measure the resistance of the LDR- you will be shown how to use it. This device provides it’s own power to the circuit, therefore no power supply or calculation is needed
-The LDR is mounted at the base of a wooden block; use a G-clamp to secure the block to the workbench.
-Position the lamp above the block, so that the light is shining directly onto the LDR
-Connect the LDR to the multimeter.
-Ensure the multimeter is correctly set to measure resistance, and that you know the unit of measurement being used.
-Switch on the lamp, and record the resistance of the LDR
-Place a sheet of tracing paper over the LDR (which may have been held in place using rubber bands or paper clips), then measure the resistance of the LDR again.
-Repeat the previous step until ten layers of paper have been placed over the LDR
Write the results for the required practical of Resistance in a LDR:
RESULTS
Draw a results table with two columns - one showing the number of layers of tracing paper (from 0-10), and the other showing the resistance (in ohms or kilo- ohms)
-Plot a graph, showing resistance (y-axis) against a number of layers of paper (x-axis); the layers of paper must be plotted in reverse order (so that the brightness increases from left to right).
-Connect the points using a smooth curve of best fit,
Write the METHOD for the required practical: Resistance in a Thermistor
METHOD
In this experiment, a digital mulitmeter will be used to measure the resistance of the thermistor- you will be shown how to use it. This device provides its own power to the circuit, therefore no power supply or calculation is needed.
1) Connect the Thermistor to the multimeter
-Ensure that the multimeter is correctly set to measure resistance, and that you know the unit of measurement being used.
-Boil the kettle, then pour hot water into a beaker.
-There should be enough water to cover the bulb of the thermometer and the thermistor.
-Immerse the thermometer and the thermistor in the hot water
-When the temperature of the water has cooled to 75 degrees celsius, record this value, with the corresponding resistance of the thermistor
-Allow the water to cool to 70 degrees celsius, then record the temperature and resistance again.
-Repeat the previous step until the water has cooled to 40 degrees celsius or cooler, taking readings at 5 degree intervals (you should have at least eight pairs of values)
Write the results for the required practical of Resistance in a Thermistor:
RESULTS
Draw a results table with 2 columns- one showing the temperature of the water (in C) and the other one showing the resistance (in ohms or kilo -ohms)
-Plot a graph, showing resistance (y-axis) against temperature (x-axis). Connect the points using a smooth curve of best fit.
Describe the trend in LDR and Thermistor graphs.
As the light intensity increases, the resistance of the LDR decreases.
As the temperature increases, the resistance of the Thermistor decreases.
In both cases, the external stimulus excites the electrons in the component and helps them to flow, increasing the current, reducing the resistance.
(Note that the resistance can never equal to zero)
